1. Field of the Invention
The present disclosure generally relates to semiconductor lasers, laser controllers, frequency converted light sources, and other optical systems incorporating semiconductor lasers. More specifically, the present invention relates to methods for aligning frequency converted light sources that include, inter alia, a semiconductor laser optically coupled to a second harmonic generation (SHG) crystal, or another type of wavelength conversion device, with adaptive optics.
2. Technical Background
Short wavelength light sources can be formed by combining a single-wavelength semiconductor laser, such as an infrared or near-infrared distributed feedback (DFB) laser, distributed Bragg reflector (DBR) laser, or Fabry-Perot laser, with a light wavelength conversion device, such as a second harmonic generation (SHG) crystal. Typically, the SHG crystal is used to generate higher harmonic waves of the fundamental laser signal. To do so, the lasing wavelength is preferably tuned to the spectral center of the wavelength converting SHG crystal and the output of the laser is preferably aligned with the waveguide portion at the input facet of the wavelength converting crystal.
Waveguide optical mode field diameters of typical SHG crystals, such as MgO-doped periodically poled lithium niobate (PPLN) crystals, can be in the range of a few microns while semiconductor lasers used in conjunction with the wavelength conversion device may comprise single-mode waveguides having diameters of approximately the same dimensions. As a result, properly aligning the beam from the semiconductor laser with the waveguide of the SHG crystal such that the output of the SHG crystal is optimized may be a difficult task which requires that the position of the beam of the semiconductor laser be precisely controlled along two axes on the input face of the SHG crystal.
Similarly, the phase matching bandwidth of SHG crystals are typically narrow, generally less than 1 nm. For example, for a 12 mm long PPLN crystal, the phase matching bandwidth may be about 0.16 nm. As such, the wavelength of the semiconductor laser must be precisely controlled to properly align the semiconductor laser with the SHG crystal and obtain the desired output intensity and wavelength from the SHG crystal.